使用stream流进行条件查询

public class Demo {
          
   
    private Integer id;
    private Integer num;

    public Integer getId() {
          
   
        return id;
    }

    public void setId(Integer id) {
          
   
        this.id = id;
    }

    public Integer getNum() {
          
   
        return num;
    }

    public void setNum(Integer num) {
          
   
        this.num = num;
    }

    @Override
    public String toString() {
          
   
        return "Demo{" +
                "id=" + id +
                ", num=" + num +
                };
    }

    public Demo(Integer id, Integer num) {
          
   
        this.id = id;
        this.num = num;
    }
}

import java.util.ArrayList;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

public class test03 {
          
   
    public static void main(String[] args) {
          
   
        List<Demo> list = new ArrayList<>();
        list.add(new Demo(1,1));
        list.add(new Demo(1,3));
        list.add(new Demo(2,2));
        list.add(new Demo(3,2));
        list.add(new Demo(3,8));
        //统计集合中符合条件的数据数量
        long count = list.stream().filter(s -> s.getId() == 1).count();
        //统计集合中符合条件的数据
        List<Demo> demoList = list.stream().filter(s -> s.getNum() == 2).collect(Collectors.toList());
        //统计集合中符合条件的第一条数据
        //Java 8 新特性 Optional 类是一个可以为null的容器对象
        Optional<Demo> demo = list.stream().filter(s -> s.getNum() == 2).findFirst();
        System.out.println(count);
        System.out.println(demoList);
        System.out.println(demo);
    }
}

运行结果如下：